Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing sides of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof synonymously refer to materials that undergo a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging). Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed.
Some active materials can be organic compounds that are capable of undergoing a reversible oxidation-reduction cycle. Organic active materials often provide relatively limited energy densities due to low solubility values, particularly in aqueous electrolyte solutions, and low electrical conductivity. To compensate for low solubility values, organic active materials are frequently used in non-aqueous electrolyte solutions so that increased solubility can be realized. High synthesis costs and environmental issues can sometimes accompany the use of organic active materials in flow batteries.
Metal-based active materials can often be desirable for use in flow batteries and other electrochemical energy storage systems. Although non-ligated metal ions (e.g., dissolved salts of a redox-active metal) can be used as an active material, it can often be more desirable to utilize coordination complexes for this purpose. As used herein, the terms “coordination complex,” “coordination compound,” “metal-ligand complex,” or simply “complex” synonymously refer to a compound having at least one covalent bond formed between a metal center and a donor ligand. The metal center can cycle between an oxidized form and a reduced form in an electrolyte solution, where the oxidized and reduced forms of the metal center represent states of full charge or full discharge depending upon the particular half-cell in which the coordination complex is present. In certain instances, additional electrons can be transferred through the oxidation or reduction of one or more of the molecules constituting the ligands.
Titanium complexes can be particularly desirable active materials for use in flow batteries and other electrochemical energy storage systems, since such metal complexes can provide good half-cell potentials (e.g., less than −0.3 V) and current efficiencies exceeding 85% at high current density values (e.g., greater than 100 mA/cm2). Various catechol complexes of titanium can be especially desirable active materials in this regard, since they are relatively stable complexes and have a significant degree of solubility in aqueous media. Although various methods are available for synthesizing catechol complexes of titanium (also referred to herein as titanium catecholate complexes or titanium catechol complexes), none are presently viable for producing the significant quantities of these complexes needed to support commercial-scale energy storage applications. In addition, concurrent production of extraneous salts during conventional syntheses of titanium catechol complexes can be especially problematic, as discussed further hereinafter.
Titanium catechol complexes are usually synthesized in a salt form, wherein the complex itself bears a formal negative charge and one or more positively charged counterions are present to maintain charge balance. Concurrent production of extraneous salts that are not associated with the titanium catechol complexes can, in many instances, undesirably decrease solubility of the complexes through a common ion effect upon forming an electrolyte solution, particularly an aqueous electrolyte solution. Introduction of excessive counterions while forming titanium catechol complexes in a desired salt form can lead to the undesirable co-production of extraneous salts. In many instances, the excessive counterions can react with a byproduct formed during the synthesis of the titanium catechol complexes and lead to production of the extraneous salts. Similarly, introduction of insufficient counterions can lead to incomplete formation of a desired salt form. Neither of these situations is optimal for forming electrolyte solutions intended to have a high energy density and other desirable parameters.
In view of the foregoing, improved methods for synthesizing titanium catechol complexes to support their use as active materials in energy storage applications would be highly desirable in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.